fix lockfile for nix 2.5

flake.lock

@@ -4,7 +4,7 @@
       "inputs": {
         "artiq-comtools": "artiq-comtools",
         "mozilla-overlay": "mozilla-overlay",
-        "nixpkgs": "nixpkgs_2",
+        "nixpkgs": "nixpkgs_3",
         "sipyco": "sipyco_2",
         "src-migen": "src-migen",
         "src-misoc": "src-misoc",
@@ -107,6 +107,22 @@
         "type": "github"
       }
     },
+    "nixpkgs_4": {
+      "locked": {
+        "lastModified": 1644472683,
+        "narHash": "sha256-sP6iM4NksOYO6NFfTJ96cg+ClPnq6cdY30xKA1iYtyU=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "7adc9c14ec74b27358a8df9b973087e351425a79",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-21.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "artiq": "artiq"
@@ -114,11 +130,7 @@
     },
     "sipyco": {
       "inputs": {
-        "nixpkgs": [
-          "artiq",
-          "artiq-comtools",
-          "nixpkgs"
-        ]
+        "nixpkgs": "nixpkgs_2"
       },
       "locked": {
         "lastModified": 1644649772,
@@ -136,7 +148,7 @@
     },
     "sipyco_2": {
       "inputs": {
-        "nixpkgs": "nixpkgs_3"
+        "nixpkgs": "nixpkgs_4"
       },
       "locked": {
         "lastModified": 1644649772,